Viral superantigens (vSAGs) are encoded by Mouse Mammary Tumor Virus (MMTV) proviral integrants. when developing thymocytes bearing T cell receptors with particular Vbeta elements encounter vSAGs as self molecules in the thymus, they are clonally eliminated. This mechanism of tolerance induction by endogenous viral superantigen (vSAG) expression will be studied using B10.BR-Mtv-l and B10.BR-Mtv-6 mice, which carry either Mtv-l or Mtv-6 proviruses on a B10.BR background. The protein products of these MMTV integrants, vSAG1 and vSAG6, both interact with Vbeta3+ T cells and have identical amino acid sequences. Interestingly, vSAG6 expression results in the complete deletion of Vbeta3+ T cells, whereas vSAG1 expression results in only partial deletion. This unique system will allow a detailed inspection of whether differences in vSAG expression levels, cell types expressing and presenting vSAG, or the developmental stage of thymocytes at the time of vSAG exposure may influence the outcome of the interaction. Preliminary data reveal significantly greater thymic vSAG6 mRNA expression in B10.BR-Mtv-6 mice than vSAG1 expression in B10.BR-Mtv-l animals, correlating with a more complete deletion of reactive thymocytes at an earlier point in the maturational sequence. Future experiments will establish the tissue distribution of vSAGs 1 and 6; firstly, by fractionation of thymic cell populations followed by quantitative RT-PCR detection of vSAG mRNA and secondly, by in situ hybridization with radiolabeled antisense riboprobes. The ability of vSAGl and 6-expressing cell types to transmit the negative selection signal will be tested in an in vitro deletion assay. Subsequent studies will address which accessory molecules, if any, are important in transmitting the apoptotic signal. The experiments described will add considerably to our understanding of the factors which mediate the negative selection of autoreactive T cells, and may ultimately provide insight into mechanisms of autoimmune disease induction.